Ultrasound transducer with curved transducer stack

ABSTRACT

A high frequency ultrasound array having a number of transducer elements that are formed in sheet of piezoelectric material. A frame having a coefficient of thermal expansion similar to that of the piezoelectric material surrounds the piezoelectric material and is separated from the piezoelectric material by an epoxy material. Kerf cuts that define the individual elements in the sheet of piezoelectric material extend across a full width of the sheet. In some embodiments, sub-dice kerf cuts that divide a single transducer element into two or more sub-elements also extend all the way across the width of the sheet. A lens positioned in front of the transducer elements can have a radius machined therein to focus ultrasound signals. The frame, transducer elements and lens are bent or curved with the desired radius to focus ultrasound signals.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is related to, and claims the benefit of, U.S.Provisional Patent Application Ser. No. 62/680,444 filed Jun. 4, 2018,which is herein incorporated by reference in its entirety

TECHNICAL FIELD

The disclosed technology relates generally to phased array ultrasoundtransducers and in particular to high frequency, ultrasound transducers.

BACKGROUND

As will be appreciated by those skilled in the art, most modernultrasound imaging systems work by creating acoustic signals from anumber of individual transducer elements that are formed in a sheet ofpiezoelectric material. By applying a voltage pulse across an element,the element is physically deformed thereby causing a correspondingultrasound signal to be generated. The signal travels into a region ofinterest where a portion of the signal is reflected back to thetransducer as an echo signal. When an echo signal impinges upon atransducer element, the element is vibrated causing a correspondingvoltage to be created that is detected as an electronic signal.Electronic signals from multiple transducer elements are combined andanalyzed to determine characteristics of the combined signal such as itsamplitude, frequency, phase shift, power and the like. Thecharacteristics are quantified and converted into pixel data that isused to create an image of the region of interest.

A phased array transducer works by selectively exciting more than oneelement in the array at a time so that a summed wave front is directedin a desired direction. By carefully changing the phase (e.g. timedelay) and in some cases, the amplitude of the signals produced by eachtransducer element, a combined beam can be directed over a range ofangles in order to view areas other than those directly ahead of thetransducer. For a phased array transducer to work well, the pitch of theindividual transducer elements is generally required to be about ½ ofthe wavelength of the center frequency of the transducer or less. Whilelow frequency, phased array transducers (e.g. 2-10 MHz) have been usedfor some time, high frequency phased array transducers have beendifficult to manufacture due to the small size of the transducerelements and the higher attenuation of high frequency ultrasoundsignals.

High frequency ultrasound (e.g. 15 MHz and higher) is an increasinglyused imaging modality that is used to image fine detail in a body and tocapture images of moving tissues. As the operating frequency of atransducer increases, the size of the transducer elements decreases.However, many manufacturing techniques that are used to make lowerfrequency phased array transducers cannot simply be scaled to createhigh frequency phased array transducers. One component of the transducerthat is expensive and time consuming to manufacture is a high frequencylens. A lens for a high frequency ultrasound transducer is typically athin sheet of material that includes a concave radius to focus theultrasound signals at a pre-determined depth. The radius is timeconsuming and expensive to put into the lens material. In addition,forming corresponding curvatures in any matching layers on the lens is atime consuming and expensive process. Given this problem, there is aneed for improving the focusing mechanism of a high frequency ultrasoundtransducer array.

SUMMARY

The disclosed technology is a high frequency ultrasound transducer arrayhaving a frame that surrounds a sheet of piezoelectric material. Theframe has coefficient of thermal expansion that is matched to thepiezoelectric material. A number of transducer elements are formed inthe sheet of piezoelectric material and one or more matching layers arecoupled to a front face of the transducer elements in order to match anacoustic impedance of the transducer elements to an acoustic impedanceof a lens that focuses the ultrasound signals produced. In oneembodiment, kerf cuts are placed in the matching layers that align withthe kerf cuts that define the individual transducer elements. The kerfcuts in the matching layers are filled with a material such as a loadedepoxy. A lens is secured to an outermost matching layer and alsoincludes a number of filled kerf cuts that are aligned with the kerfcuts in the one or more matching layers. In one embodiment, one or moreadditional matching layers are added to a front surface of the lens tomatch an acoustic impedance of the lens material to the acousticimpedance of water.

In one embodiment, the transducer stack including frame, piezoelectricmaterial and lens are bent to have a concave radius in order to focusultrasound signals from the transducer elements. The transducer stackcan be adhered to a curved support frame to hold the transducer stack inits curved configuration. In another embodiment, a curved bezel issecured to the transducer stack to maintain the curvature of the stack.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a frame that surrounds a piezoelectricsheet in accordance with an embodiment of the disclosed technology;

FIG. 2A is an isometric view of a frame surrounding a piezoelectricsheet and that is filled with an epoxy material in accordance with anembodiment of the disclosed technology;

FIG. 2B an isometric, cross-sectional view of the frame andpiezoelectric sheet shown in FIG. 2A;

FIG. 3 illustrates how transducer element kerf cuts and sub-dice kerfcuts are fashioned in the piezoelectric sheet with a laser in accordancewith an embodiment of the disclosed technology;

FIG. 4 is an enlarged, cross-sectional view of a piezoelectric sheetshowing a filler epoxy placed in the transducer element kerf cuts andthe sub-dice kerf cuts;

FIG. 5 is an enlarged, cross-sectional view of a portion of a transducerstack showing a number of matching layers applied to a front face of atransducer layer that are diced and filled with an epoxy material and alens that is bonded to the matching layers in accordance with anembodiment of the disclosed technology;

FIG. 6A is a partial isometric view of a lens and matching layers placedover the transducer stack;

FIG. 6B is a close-up view of a number of ridges formed in a topmatching layer that support a lens in accordance with some embodimentsof the disclosed technology;

FIG. 7 shows a number of alternative sub-dice kerf cut patterns for apiezoelectric layer in accordance with embodiments of the disclosedtechnology;

FIG. 8 shows a number of alternative sub-dice kerf cut patterns for anumber of matching layers in accordance with embodiments of thedisclosed technology;

FIG. 9 shows a conductive support frame secured to a transducer stack inaccordance with an embodiment of the disclosed technology;

FIG. 10 shows an alternative embodiment of a frame that surrounds apiezoelectric sheet in accordance with the disclosed technology;

FIGS. 11 and 12 are cross-sectional views of a high frequency phasedarray transducer in accordance with an embodiment of the disclosed;

FIG. 13 shows an alternative embodiment of a frame that surrounds apiezoelectric sheet that allows the transducer stack to be curved inaccordance with another embodiment of the disclosed technology;

FIG. 14 is a cross-sectional view of a mold that can be used to bend atransducer stack in accordance with an embodiment of the disclosedtechnology;

FIG. 15 is a cross-sectional view of a high frequency phased arraytransducer with a curved transducer stack in accordance with anembodiment of the disclosed technology; and

FIG. 16 illustrates a curved bezel that can be used to maintain thecurvature of a transducer stack in accordance with an embodiment of thedisclosed technology.

DETAILED DESCRIPTION

As will be described in detail below, a high frequency, ultrasoundtransducer includes a sheet of piezoelectric material that is surroundedby a frame. The frame is made from an electrically conductive ornon-conductive material having a coefficient of thermal expansion (CTE)that is similar to the CTE of the sheet of piezoelectric material. Theframe surrounds the piezoelectric material and is separated from thepiezoelectric material by an insulating material such as an epoxy. Kerfcuts that define the individual transducer elements in the sheet ofpiezoelectric material extend across a full width of the piezoelectricsheet. In some embodiments, sub-dice kerf cuts divide a singletransducer element into two or more sub-elements. In some embodiments,the sub-dice kerf cuts are parallel to the kerf cuts that define theindividual transducer elements. In other embodiments, the sub-dice kerfcuts are cut at an angle or perpendicular to the transducer element kerfcuts to create a 1 3 composite. For example, 90 degree kerfs can be cutto create square or rectangular piezoelectric pillars in thepiezoelectric sheet.

In previous generations of high frequency ultrasound transducers, asheet of piezoelectric material was laser machined to create a number ofkerf cuts that define individual transducer elements. The open space inthe kerf cuts that define adjacent transducer elements and the sub-dicekerf cuts within an element were filled with an epoxy material beforethe sheet was lapped to a desired thickness. The kerf cuts had a lengththat was less than a width of the piezoelectric sheet so that a borderor perimeter of the piezoelectric material provided some strength aroundthe transducer elements.

While this approach works well, it is believed that improvements can bemade. For example, the curing epoxy in the kerf cuts places thepiezoelectric sheet under stress. Because the epoxy shrinks when itcures, each transducer element is pulled sideways into a kerf cut. Whilenot significant for one or two elements, the stresses summed over allthe elements may reach a level where the piezoelectric sheet can crack.In addition, it is believed that stresses caused by the shrinking theepoxy deform the transducer elements to create a constant stress bias onthe transducer. Finally, because each transducer element is physicallyjoined to the others at the perimeter of the sheet, there is some levelof cross-talk between the elements as they are excited with a drivingpulse and as the echo signals impinge upon the elements. As will bedescribed below, one aspect of the disclosed technology is a transducerarray design where the kerf cuts extend across an entire width of thepiezoelectric sheet. Preliminary simulations indicate that the discloseddesign not only reduces the stresses caused by the curing of the kerffilling material but also reduces coupling between the transducerelements. This is supported by experimentation showing a significantimprovement in bandwidth and sensitivity over the previous kerf designsthat extend for less than the full width of a piezoelectric sheet.

As will be appreciated by those skilled in the art, the embodimentsshown in the Figures are drawn for the purpose of explaining how to makeand use the disclosed technology and are not necessarily drawn to scale.

As shown in FIG. 1, a transducer stack includes a frame 10 into which asheet of piezoelectric material is placed. The frame 10 has a centralopening 12 that receives the sheet of piezoelectric material and createsa space between an outer edge of the piezoelectric sheet and an interioredge of the frame 10. The frame 10 is preferably made of a materialhaving a coefficient of thermal expansion (CTE) that is similar to theCTE of the piezoelectric material. In some embodiments, the sheet ofpiezoelectric material is made from lead zirconate titanate, morecommonly known as PZT. For the remainder of the description, thepiezoelectric material is described as PZT. However, it will beappreciated that other materials such as single crystal ferroelectricrelaxors (e.g. PMN-PT) or synthetic piezoelectric materials could besubstituted for PZT. In the case of PZT, one suitable choice for theframe material is alumina, which is a non-conductive ceramic with a CTEthat is close to the CTE of PZT. Alumina has a CTE of about 7.2microns/M° C. where the CTE for PZT is approximately 4.7 microns/M° C.However, it will be appreciated that other materials with a coefficientof thermal expansion similar to that of the piezoelectric material couldbe used such as molybdenum or fine grain isotropic graphite. For thepurposes of the present application, coefficients of thermal expansionare similar if the piezoelectric material in the frame doesn't crack dueto thermal stresses when operated and handled over its normaltemperature operating range. In some embodiments, the frame 10 mayinclude a number of inwardly extending tabs or fiducials 14 (shown indashes) that center the PZT in the opening and space the PZT from theinside edge of the frame. In some embodiments, the tabs 14 are used ifthe frame 10 is made of a conductive material like molybdenum orgraphite but are not used if the frame 10 is made of non-conductivealumina.

FIGS. 2A and 2B show a sheet of piezoelectric material 20 placed in theopening 12 of the frame 10. The space between the interior edge of theframe 10 and outer edges of the sheet of piezoelectric material 20 isfilled an insulating filling material 24. In one embodiment, the fillingmaterial 24 is an epoxy such as from the EPO-TEK 301 family availablefrom Epoxy Technology, Inc, Billerica Mass. that is doped with hafniumoxide or ceramic particles. The particles are added to the epoxy toresist shrinkage and to resist laser machining as described below. Inthe embodiment shown in FIG. 3, the filling material 24 is molded aroundthe sides of the sheet of piezoelectric material 20 and is flush with atop surface of the sheet piezoelectric material 20 to form a stack 30having a top surface 32 and a bottom surface 34. In the descriptionbelow, the bottom surface 34 of the stack faces towards the region ofinterest and the top surface 32 faces proximally toward the ultrasoundoperator in a finished transducer.

Once the filling material 24 is cured, the top surface 32 and the bottomsurface 34 of the stack 30 are lapped, ground or otherwise made flat toremove any extra epoxy and to provide flat references for a number ofadditional machining steps as will be described.

With the top and bottom surfaces lapped, kerf cuts are created in thePZT sheet 20 with an excimer or other patterning laser. As shown in FIG.3, kerf cuts 40 are cut across the entire width of the PZT sheet 20 fromone edge to the other. If the frame 10 includes the alignment tabs orfiducials 14, the kerf cuts begin at positions away from each end of thePZT sheet to define inactive regions 42 and 44 that are located near thealignment tabs 14. In this way, the ends of the transducer elements areseparated from the interior edge of the frame 10 by an epoxy filled gap.If the alignment tabs 14 are not used, then the entire PZT sheet can bediced to form transducer elements. Because the epoxy of the fillingmaterial 24 is softer than the PZT, the transducer elements areeffectively floating in the cured filling material 24. As indicatedabove, the kerf cuts that define individual transducer elements canbegin in the filling material on one side of the frame and continueacross the entire width of the PZT sheet 20 to the filling material 24on the other side of the PZT sheet.

In one embodiment, the kerf cuts are placed at a desired pitch and to adepth sufficient to form the transducer elements depending on thedesired center frequency of the transducer being manufactured. In someembodiments, a transducer element comprises two electrically connectedsub-elements that are separated by a sub-dice kerf cut that extendsacross the entire width of the PZT. In one embodiment, the sub-dice kerfcuts have the same depth as the kerf cuts that define individualtransducer elements. However, the sub-dice kerf cuts could be cut to ashallower depth than the primary kerfs such that they do not extend allthe way through the final thickness of the PZT. In other embodiments,the transducer elements may not include any sub-dice kerf cuts.

After the kerf cuts that define the transducer elements and the sub-diceelements (if used) are fashioned by the laser, the kerf cuts are filledwith an epoxy material 48 as shown in FIG. 4. In one embodiment, theepoxy material used to fill in the kerf cuts is a doped flexible EPO-TEK301 epoxy.

In one embodiment, the epoxy material 48 is applied to the part undervacuum so that no air is trapped in the bottom of the kerfs cuts. Aliquid epoxy is applied and the part is then put under relatively highpressure (e.g. 100+ psi) to drive the liquid epoxy into the kerf cutsand allowed to cure.

After the epoxy 48 in the kerf cuts has cured, the bottom surface 34 ofthe stack is lapped, ground or otherwise made flat. Next, a groundinglayer 60 of a conductive metal such as gold or gold+an adhering metalsuch as chromium is applied to the front face of the stack by sputteringor a similar technique. The conductive grounding layer 60 covers a frontface of the diced PZT, a front face of the frame 10 and a front face ofthe epoxy filling material 24 that lies between the frame 10 and theedge the PZT sheet (as viewed when the transducer is in use).

After the conductive grounding layer 60 is applied, one or more matchinglayers M1, M2 and M3 (in the embodiment shown) and a lens L1 are appliedto the front surface of the stack as shown in FIG. 5. The number ofmatching layers used depends on the mismatch between the acousticimpedance of the PZT and the acoustic impedance of the lens material. Inthe illustrated embodiment, three matching layers M1, M2 and M3 are usedon the front surface of the stack. In one embodiment, each of thematching layers comprises an epoxy material that is doped with powdersto alter its acoustic performance in order to achieve a requiredtransducer performance.

In one embodiment, matching layer M1 that is applied over the conductiveground layer 60 comprises a layer of EPO-TEK 301 epoxy doped withtungsten powder.

In one embodiment, matching layer M2 is applied over the surface ofmatching layer M1 and comprises a layer of EPO-TEK 301 epoxy doped withtungsten powder and silicon carbide (SIC) nanoparticles.

In one embodiment, matching layer M3 is applied over the surface ofmatching layer M2 and comprises a layer of EPO-TEK 301 epoxy doped withsilicon carbide (SIC) nanoparticles.

In one embodiment, each of the matching layers has a thickness that ispreferably an odd multiple of a ¼ wavelength at the center operatingfrequency of the transducer. Most often, the thicknesses will be one of1, 3, 5 or 7 quarter wavelengths thick. However, this may vary dependingon the desired acoustic properties of the transducer. It will beappreciated that these matching layers are merely exemplary and thatother matching layer compositions may be used depending on the desiredoperating frequency of the transducer, the lens material to be used etc.The details of how matching layers can be doped with particles toachieve a desired acoustic impedance are considered to be known to thoseof ordinary skill in the art of ultrasound transducer design.

After each matching layer is applied and cured, the front face of thestack is lapped to achieve a desired thickness and to keep the frontsurface flat. In some embodiments, kerf cuts 62 are cut in the curedmatching layers with a laser to align with both the kerf cuts thatdefine the individual transducer elements and the sub-dicing kerf cuts(if used). In other embodiments, kerf cuts 62 can be made in thematching layers to align with only the kerf cuts that define theindividual transducer elements and not over the sub-dice kerf cuts. Inone embodiment, the kerf cuts 62 extend through the matching layersM3-M1 and may extend partially or fully through the grounding layer 60with no loss of connectivity between the grounding layer and thetransducer elements. Once created, the kerf cuts 62 in the matchinglayers are filled with the same filled epoxy material that fills thekerf cuts in the PZT material.

After the matching layers are kerf cut, the lens material is bonded tothe matching layers. In a high frequency phased-array, kerf cuts 96 areformed in the lens 80 that are aligned with the kerf cuts 62 in thematching layers (including matching layers M4 and M5 disposed on thefront of the lens) as shown in FIG. 6A. In some embodiments, the lens 80includes kerf cuts 96 that are aligned with both the transducer elementkerf cuts and the sub-dice kerf cuts. In other embodiments, the lensonly includes kerf cuts that are aligned with the kerf cuts that definethe individual transducer elements. In some embodiments, kerf cuts aremade in the lens material and filled with a doped epoxy prior tomounting the lens to the matching layers. In other embodiments, the lenscan be bonded first and then kerf cut and filled. A curvature 98 ismachined into the front side of the lens so that the lens focuses theultrasound in a plane at a desired depth.

In some embodiments, depressions or indentations 72 are laser machinedinto the uppermost matching layer (e.g. M3) at positions between thefilled kerf cuts 62 thereby forming a number of beams or ridges 74 atthe top of the filled kerfs that extend across the width of the PZTsheet as best shown in FIG. 6B. The beams 74 support the lens 80 acrossthe width of the PZT sheet. It is believed that the beams 74 helpmaintain a consistent distance between the rear surface of the lens 80and the uppermost matching layer as the lens is being secured to thestack whereas without the beams/ridges the distance between a centerarea and the uppermost matching layer may vary as the two are beingsecured together if the lens 80 is only supported around a perimeter ofthe lens. In high frequency arrays where the tolerances are very tight,having the beams/ridges 74 may help ensure a consistentlens-to-transducer distance over the entire area of the active elements.If the uppermost matching layer does not include filled kerf cuts, theridges 74 can be formed anywhere in the matching layer and not betweenthe filled kerf cuts in the lower matching layers.

In one embodiment, the same material used for the uppermost matchinglayer M3 is used to glue the lens 80 to the stack. Because theindentations 72 are shallow and the same material used for the uppermostmatching layer M3 is used as a glue to secure the lens 80 to thematching layer, there is minimal acoustic discontinuity at the bondline.

For a transducer to operate well as a phased-array, the beam pattern ofthe energy produced by each element at the front face of the lens has tobe sufficiently wide so that the lateral components can combine with thelateral components of the beams from adjacent elements to effectivelysteer the beam. In one embodiment, the beam patterns have energy at+/−45 degrees from normal that exceeds −6 dB in signal power.

For low frequency transducers, lens materials are available that allowsuch transducers to be made. However, at high frequencies, the physicalproperties of such materials make them unacceptable for transducerdesigns, For example, silicone materials are often used as a lensmaterial for low frequency transducers due to their close acousticimpedance to water. However, the absorption of ultrasound in siliconeincreases exponentially with frequency and at 15 MHz+, the absorption inthe material is too great to allow it to be used as an effective lens.To overcome such absorption, harder materials are often used for highfrequency ultrasound lenses such as polymethylpentene (sold under thetradename TPX) and cross-linked polystyrene (sold under the tradenameRexolite). While acceptable for non-phased arrays, such materials aredifficult to use in phased arrays because of Snell's law.

With Snell's law, energy passing from a faster material to a slowermaterial tends to bend towards a line normal to the interface. This isprecisely the wrong direction for a phased array when it is desired thata portion of the beam energy extend at an angle away from normal. Tocompensate for the Snell's law effect, energy has to be supplied at agreater angle of incidence that quickly approaches a critical angle ofthe lens material where all the energy is reflected internally. In anultrasound transducer, internally reflected energy from one transducerelement can cause spurious signals to be generated at neighboringelements. In addition, phase aberrations associated with such internalreflections make it nearly impossible to perform beamforming withsignals from multiple adjacent transducer elements.

To solve these problems, a phased array transducer design in accordancewith the disclosed technology includes a lens configured so that it hasan anisotropic speed of sound in the forward direction compared to asideways or lateral direction in the lens. In one embodiment, a lens ismade from a material having a speed of sound that is much faster thanthe speed of sound in the material that fills the kerf cuts in the lens.As shown in FIG. 6A, a lens 80 includes a sheet of polybenzimidazole(sold under the tradename Celazole™). The lens (and outer matchinglayers if used) is patterned with a laser to form a number of kerf cuts96 that can align with the kerf cuts defining individual transducerelement or the kerf cuts defining individual transducer elements and thesub-dice kerf cuts. Celazole is useful as a high frequency lens materialbecause it has a high speed of sound, and because it can withstand theheat of the laser used when making the kerf cuts at a fine pitch (e.g.at 40 um or less for a 20+ MHz phased array) without melting. Inaddition, Celazole can be bonded to the epoxy of the uppermost matchinglayer of the transducer stack directly so that no cyanoacrylate (CA)layer of glue is necessary.

In one embodiment, the kerf cuts 96 in the lens 80 and outermostmatching layers are filled with a material having a much slower speed ofsound than the speed of sound of the lens material such as a powderfilled or doped epoxy or RTV 60 silicone.

In some embodiments, the lens 80 also includes one or more matchinglayers M4 and M5 on its front surface that match the impedance of thelens material to water. M4 and M5 are formed to have a curvature thatmatches and is aligned with the curvature of the lens. In someembodiments, two powder-filled or doped epoxy matching layers (M4, M5)are applied to the front surface of the lens. In one embodiment, thematching layers on the lens are applied on mandrel having the sameradius of curvature as the radius or curvature that is machined into thelens material. In some embodiments, the outer matching layers M4, M5 arediced with the laser to continue the kerf cuts 96 formed in the lensmaterial 80 and filled with the same material that fills the kerf cutsin the lens material. In other embodiments, the matching layers on thefront of the lens 80 may omit the kerf cuts on one or both of M4 and M5.

With this construction, the lens material between the kerf cuts 96 inthe lens 80 forms a number of mini-waveguides that channel the energy ofthe ultrasound transducer elements in a direction straight ahead withless energy spreading laterally in the lens. In one embodiment, thespeed of sound through the lens in the axial direction of the transducerstack is greater than the speed of sound in a direction sideways throughthe lens or in a direction parallel to a front face of the lens.

In some embodiments, additional kerf cuts can be machined into the PZTlayer in addition to those defining the individual transducer elements.FIG. 7 illustrates a number of possible sub-dicing patterns. A pattern150 is a conventional sub-dice pattern where a transducer element isdivided lengthwise down its center by a single sub-dice kerf cut. Thissub-dice kerf cut has the same length as the transducer element. As willbe appreciated by those skilled in the art, the width/height ratio of atransducer element should be less than or equal to the “golden ratio” ofabout 0.6 to minimize lateral vibrational modes in the PZT. In someembodiments of the disclosed technology, an excimer UV laser can cut akerf line of approximately 6 um in width. At a 40 micron element pitchand 70-80 micron PZT thickness, this ratio can be met without using acenter sub-dice kerf.

Other sub-dice patterns may be useful for certain transducerapplications. A pattern 154 includes a number of parallel sub-dice kerfcuts that are cut at an acute angle (e.g. 55 degrees) with respect tothe kerf cuts that define the transducer elements. In the embodimentshown, the parallel sub-dice kerf cuts are spaced 28 microns apart butother spacings could be used.

A third sub-dice pattern 158 is formed by alternating sets of parallelcuts that are acute with respect to the direction of the kerf cuts thatdefine the transducer elements. The result is a set of alternatelyoriented, triangular PZT pillars each having a base that is aligned witha kerf cut defining the transducer element and a height that is thewidth of the transducer element. In the embodiment shown, each suchtriangle has a base that is 56 microns wide and a height of 40 micronsfor a transducer with elements at a 40 micron pitch.

A fourth pattern 162 is made with sub-dice kerfs cuts that areperpendicular to the kerf cuts that define the transducer elements. Inthis pattern, a number of rectangular PZT pillars are formed with aheight of, for example, 28 microns and width equal to the width of thetransducer elements (40 microns in the embodiment shown).

A fifth pattern 166 is made with sub-dice kerf cuts that are formed by aplurality of parallel cut kerf cuts oriented at an acute angle (e.g. 45degrees) with respect to the kerf cuts defining the individualtransducer elements and that are interspaced with kerf cuts that areperpendicular to the kerf cuts that define the individual transducerelements. This pattern forms a number of alternating right triangleswith their hypotenuses facing each other in the transducer element. Inthe embodiment shown, the legs of the right triangles are 40 micronslong.

A sixth pattern 170 of kerf cuts forms a number of alternately orientedequilateral triangles in the transducer element by forming kerf cuts at60 degrees with respect to the kerf cuts that define the individualtransducer elements.

FIG. 8 shows a number of possible sub-dice kerf cuts that can be formedin the matching layers (M1-M5) to correspond to the sub-dice kerf cutsin the PZT layer.

A pattern 180 corresponds to the pattern 150 with a single kerf cutdefining a pair of sub-diced elements. A pattern 182 corresponds to theright triangular pattern 166. A pattern 184 corresponds to thealternating triangular pattern 158, while a pattern 186 corresponds tothe alternating equilateral triangle pattern 170.

As indicated above, in some cases the matching layers include kerf cutsthat match the sub-dice kerf cuts in the PZT layer. In otherembodiments, the matching layers include fewer than all the sub-dicekerf cuts in the PZT layer and may only include kerf cuts matching thekerf cuts that define the individual transducer elements.

As indicated above, with the lens 80 bonded to the transducer stack, therear or proximal side of the transducer stack can be manufactured. Firstthe PZT layer and frame are lapped to a desired thickness depending onthe desired operating frequency of the transducer. Then, as best shownin FIG. 9, a conductive support frame 160 is secured with a conductiveepoxy adhesive to the rear or proximal side of the transducer stack 150.The frame 160 is preferably conductive and is made of a metal having aCTE that is similar to the PZT such as molybdenum. In the example shown,the transducer stack 150 has an elevation dimension of 4.5 mm and anazimuthal or width dimension of 7.6 mm. The support frame has an openarea 162 through which the rear surface of the individual exposed PZTtransducer elements are accessible. The support frame 160 supports oneor more flexible circuits 170 having traces (not shown) that deliverelectrical signals to and from the transducer elements. In oneembodiment, a first flexible circuit has traces connected to all theeven numbered transducer elements and a second flexible circuit onopposite side of the frame 160 (not shown) has traces connected to allthe odd numbered transducer elements. In some embodiments, a singleflexible circuit includes traces for both the even and odd transducerelements.

A ground plane in the flex circuit (not shown) is electrically connectedto the conductive support frame 160 on the back side of the transducerwith a conductive epoxy or the like. The support frame 160 thereforeacts as part of a conductive path between the common ground electrode onthe front side of the transducer stack and the ground plane in the flexcircuits. If the frame surrounding the PZT sheet is conductive (e.g.graphite or molybdenum) then the frame itself becomes part of theconductive path. If the frame is non-conductive (e.g. alumina) then aconductive path is included between the frame and the common groundelectrode on the front of the transducer stack. FIG. 10 shows anembodiment of a frame 10 having a pair of cut outs 200 along its longdimension that can be filled with a conductive epoxy to form a conductorbetween the front of the frame and the rear of the frame that in turnconnects to the conductive support frame 160. Other mechanisms forproviding the conductive path could include vias or conductive foils,wires or the like that connect the common ground electrode on the frontof the PZT elements and the conductive support frame 160 or the groundelectrodes in the flex circuits.

Once the conductive support frame 160 is secured to the transducerstack, electrical pathways are then made between exposed portions of thetraces in the flex circuits and the individual transducer elements.

In one embodiment, conductive pathways are formed on the proximal sideof the transducer elements to allow traces in flexible connectors 170(e.g. flex circuits) to be electrically connected to the transducerelements. In some embodiments, the pathways are created by filling theframe with a particle filled epoxy, creating channels in the epoxy froma transducer element to a corresponding trace in the flex circuit with alaser and by plating the channels with gold or gold plus chromiumfollowed by removing the gold in areas where it is not wanted andcleaning up the paths with the laser. Descriptions of suitablepatterning processes used to create the conductive paths between thetransducer elements and the traces in the flex circuits in accordancewith some embodiments of the disclosed technology are found in commonlyowned U.S. Patent Publication No. 2017-0144192 A1 and U.S. Pat. No.8,316,518, which are herein incorporated by reference in theirentireties.

Once the connections have been made between the transducer elements andthe traces in the flex circuits, a backing layer (not shown) is securedto the assembly behind the transducer elements. FIGS. 11 and 12 arecross-sectional views of a transducer assembly showing a transducerstack with a PZT layer, a tapered support frame surrounding the PZTlayer and a lens coupled to the stack through a number of matchinglayers. In the embodiment shown, the lens has a radius of curvature of17 mm that is machined into the lens material before the lens matchinglayers are applied. Two matching layers (M4, M5) are applied to thefront surface of the lens. In one embodiment, the matching layers on thelens are applied on mandrel having the same radius of curvature as theradius that is machined into the lens material. The tapered supportframe supports the flex circuits that provide signals to and receivesignals from the transducer elements.

Although the disclosed embodiments show element spacings that aresuitable for a high frequency phased array, it will be appreciated thatthe structure of the transducer including a piezoelectric sheet,surrounding frame, matching layers and lens could be used for non-phasedarray transducers or lower frequency transducers. In addition, if usedat lower frequencies, then other lens materials such as TPX or Rexolitecould be used.

In some embodiments, the step of forming a radius in the front face ofthe lens material to set its focal distance can be fully or partiallyavoided by bending the transducer stack to have a desired a radius inthe elevation direction. FIG. 13 shows one embodiment of a flexibleframe 220 that can surround a piezoelectric sheet 230 in a manner thatallows the transducer stack to be curved after assembly. In thisembodiment, the frame 220 includes a pair of side rails 222, 224 made ofalumina or other material with a CTE that is similar to that of PZT(e.g. molybdenum, graphite). The side rails 222, 224 may include tabs orfiducials (not shown) that space a PZT sheet 240 from an inner edge ofthe side rails if the side rails are conductive. In embodiment, the siderails do not extend around the complete perimeter of the piezoelectricsheet but are open on two opposing sides to allow a flexible adhesive250 such as an epoxy to fill the gap between the frame 220 and the PZTsheet 240. If the frame is made of a metal (e.g. molybdenum) that can bebent, then the frame may extend around the entire perimeter of thepiezoelectric sheet. In this embodiment, the transducer stack is builtby laser machining the PZT to form the transducer elements and sub-diceelements (if used), adding the grounding electrode and the one or morematching layers and the lens as described above.

Once the transducer stack is formed, it can be curved to have a desiredradius by pressing the stack between elements of an appropriately shapedmold 260 as shown in FIG. 14. In this embodiment, the mold has onesection 262 with a convex-shaped curvature and a second section 270 witha correspondingly shaped concave curvature 272. The transducer stack canbe pressed between the two sections of the mold while under heat tosoften the epoxies and bend the transducer stack to have thecorresponding shape. If the PZT layer is thin (such as, but not limitedto 80 microns or less), the transducer elements of the PZT layer willbend without cracking. Once cooled, the two sections 262, 270 of themold are removed and the transducer stack will retain the desiredcurvature in the elevation direction of the lens to focus the ultrasoundsignals at the desired distance.

In the embodiment shown, the transducer stack is only bent in theelevation direction. However, it will be appreciated that the stackcould also be bent to have a compound concave curve one or both of theelevation and azimuthal directions. Similarly, in some embodiments, itmay be desirable to put a convex curve into the transducer stack in oneor both of the elevation or azimuthal directions.

In one embodiment, a curved conductive frame that supports the flexcircuits is used as a structural support on the back side of the stackto maintain the desired curvature as shown in FIG. 15. The curved framecan either by applied after the stack is curved or can be applied to therear element side at the time of molding.

FIG. 15 is a cross-sectional view of an ultrasound transducer with atransducer stack that is curved as described. In this embodiment, asupport frame is adhered to the back surface of the transducer stack tosupport flex circuits with traces that connect to the individualtransducer elements. In one embodiment, the support frame has a curvedshape so that it holds the curved shape of the transducer stack as thestack is pressed against the curved support. In some embodiments, acurved bezel (e.g. a metal of molded ceramic frame) as shown in FIG. 16may be fixed to the front of the transducer around a perimeter of thelens to help maintain the curvature of the transducer stack.

In another embodiment, a filling epoxy fills the gaps between a curvedtransducer stack and a flat support frame on the rear surface of thestack. The filling epoxy maintains the curved shape of the stack once itis cured. Once the conductive support frame is secured to the transducerstack, connections between the individual transducer elements and thetraces in the flex circuits are made as described above.

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from scopeof the invention. For example, the disclosed transducer design can bescaled to operate at lower frequencies (e.g. 2-15 MHz). In addition,aspects of the disclosed technology can be used in more conventionalultrasound transducer designs. Accordingly, the invention is not limitedexcept as by the appended claims.

I/We claim:
 1. An ultrasound transducer, comprising: a piezoelectricmaterial having a coefficient of thermal expansion; a frame surroundingan outer perimeter of the piezoelectric material, where the frame has acoefficient of thermal expansion that is similar to a coefficient ofthermal expansion of the piezoelectric material; a filling materialplaced between the frame and the outer perimeter of the piezoelectricmaterial; wherein the piezoelectric material includes a number of kerfcuts that define a number of individual transducer elements; and a lenssecured to the piezoelectric material through one or more matchinglayers, wherein the frame, piezoelectric material and lens have aconcave curvature that is configured to focus ultrasound signalsproduced from the transducer elements.
 2. The ultrasound transducer ofclaim 1, wherein the lens and the one or more matching layers includefilled kerf cuts that are aligned with the kerf cuts in thepiezoelectric material.
 3. The ultrasound transducer of claim 1, whereinthe frame includes a pair of side rails and a filler materialsurrounding the piezoelectric material such that the filler material canbe bent into the concave curvature.
 4. The ultrasound transducer ofclaim 3, wherein the frame is made of alumina.
 5. The ultrasoundtransducer of claim 4, wherein the frame is open on opposing sides andis filled with an epoxy that allows the frame to be bent into theconcave curvature.
 6. The ultrasound transducer of claim 1, wherein theframe is made of graphite.
 7. The ultrasound transducer of claim 1,wherein the frame is made of molybdenum.
 8. The ultrasound transducer ofclaim 1, further comprising a conductive support frame having a curvedshape and is secured to the piezoelectric material to maintain theconcave curvature shape of the piezoelectric material.
 9. The ultrasoundtransducer of claim 1, further comprising a conductive support framehaving a flat surface that is secured to the piezoelectric material withan adhesive to fill a curved gap to between the curved piezoelectricmaterial the support frame to maintain the concave curvature shape ofthe piezoelectric material.
 10. The ultrasound transducer of claim 1,wherein the curvature in the piezoelectric material is in an elevationdirection of the transducer.
 11. The ultrasound transducer of claim 1,wherein the curvature in the piezoelectric material is in an azimuthaldirection of the transducer.
 12. The ultrasound transducer of claim 1,wherein the curvature in the piezoelectric material is in an elevationand azimuthal direction of the transducer.
 13. A phased array ultrasoundtransducer, comprising: a piezoelectric material having a coefficient ofthermal expansion; a frame surrounding an outer perimeter of thepiezoelectric material, where the frame has a coefficient of thermalexpansion that is similar to a coefficient of thermal expansion of thepiezoelectric material; a filling material placed between the frame andthe outer perimeter of the piezoelectric material; wherein thepiezoelectric material includes a number of kerf cuts that define anumber of individual transducer elements; and a lens secured to thepiezoelectric material through one or more matching layers, wherein thelens includes a number of kerf cuts that align with the kerf cuts in thepiezoelectric material and wherein the frame, piezoelectric material andlens have a curvature that is configured to focus ultrasound signalsproduced from the transducer elements.
 14. The phased array ultrasoundtransducer of claim 13, wherein the lens includes a number of kerf cutsthat align with the kerf cuts in the piezoelectric material that definethe individual transducer elements.
 15. The phased array ultrasoundtransducer of claim 13, wherein the frame includes a pair of side railsand a filler material surrounding the piezoelectric material such thatthe filler material can be bent into the concave curvature.
 16. Thephased array ultrasound transducer of claim 15, wherein the frame ismade of alumina.
 17. The phased array ultrasound transducer of claim 16,wherein the frame is open on opposing sides and is filled with an epoxythat allows the frame to be bent into the concave curvature.
 18. Thephased array ultrasound transducer of claim 13, wherein thepiezoelectric material has a concave bend in an axial direction of thetransducer.
 19. The phased array ultrasound transducer of claim 13,wherein the piezoelectric material has a concave bend in an azimuthaldirection of the transducer.
 20. The phased array ultrasound transducerof claim 13, wherein the piezoelectric material has a concave bend in anaxial and azimuthal direction of the transducer.